Among several candidate “next-generation lithography” technologies for use in the manufacture of semiconductor integrated-circuit devices, displays, and other highly miniaturized devices is “extreme ultraviolet lithography” (EUVL). EUVL is lithography performed using a wavelength of electromagnetic radiation in the range of 11 to 14 nm, which is within the “extreme ultraviolet” or “soft X-ray” portion of the electromagnetic spectrum. EUVL offers prospects of greater image resolution than are currently obtainable using “optical” lithography, of which the shortest wavelengths currently in use are in the range of 150-250 nm.
A current challenge in the development of a practical EUVL system is providing a convenient source of EUV exposure “light” capable of providing an EUV beam at sufficient intensity at the desired wavelength for making lithographic exposures at an acceptable throughput. The most powerful source of EUV light currently available is synchrotron radiation. Unfortunately, very few fabrication plants at which EUVL would be performed have access to a synchrotron, which is extremely large and extremely expensive to install and operate. As a result, substantial research and development effort is currently being directed to the development of alternative sources of EUV light. The two principal approaches in this development involve the production of a plasma of a target material, wherein the plasma produces EUV radiation. In one method the plasma is produced by electrical discharge in the vicinity of the target material, and in the other method the plasma is produced by laser irradiation of the target material. The EUV radiation produced by both methods is pulsed. Whereas these methods have advantages of portability as well as relatively compact size and low cost of operation (especially relative to a synchrotron), they have several disadvantages. One disadvantage is the difficulty of producing a sufficiently intense beam of EUV light at the desired wavelength for desired high-throughput exposures. Another disadvantage is that the respective plasmas produced by these sources tend to generate gases and fine debris that deposit on nearby components, especially nearby optical components. In view of the extremely high performance demanded of EUV-optical elements, significant contamination of them by debris and gases from the EUV source simply cannot be tolerated.
Because no materials are known that are sufficiently transmissive and refractive to EUV light to serve as EUV lenses, EUV optics comprise reflective optical elements (i.e., mirrors). Except for grazing-incidence mirrors, all EUV mirrors have a respective surficial multilayer film that provides the mirror surfaces with a useful reflectivity to incident EUV light. For EUVL, these mirrors must be fabricated to extremely demanding tolerances and must exhibit extremely high optical performance.
Since EUV light is greatly attenuated and scattered by the atmosphere, the propagation pathway for EUV radiation in an EUVL system is evacuated to a vacuum. This requires that the EUVL optics (e.g., illumination optics and projection optics) be contained in at least one vacuum chamber that is evacuated to a desired vacuum level. Similarly, a plasma EUV source as summarized above is contained in a vacuum chamber (termed an “EUV-source chamber”) that is evacuated to a desired vacuum level. Hence, EUV light generated in the plasma EUV source must propagate from the EUV-source chamber to the chamber containing the EUVL optics.
In the plasma EUV source, EUV light and other wavelengths of light produced by the plasma are collected into a beam. Light collection can be achieved using, for example, one or more collector mirrors situated near the plasma. From the collector mirror(s), the beam passes through the intermediate focus plane of the collector mirror(s), between the source and downstream EUV optics. From the intermediate focus plane the beam is directed as an “illumination beam” to an illumination unit (“illumination-optical system”) contained in an illumination-unit chamber. The illumination-optical system, which is part of the EUVL optics, comprises various mirrors that collectively direct, shape, and condition the illumination beam as required for illumination of a pattern-defining reticle or other “pattern master” situated downstream of the illumination-optical system. Along this beam path the beam passes through a spectral purity filter (SPF). The SPF may be located near the intermediate focus plane, or it may be located within the illumination-optical system.
The SPF is utilized because the beam collected from the plasma contains various wavelengths of EUV radiation as well as longer wavelengths of electromagnetic radiation such as infrared light, ultraviolet light, and visible light. Wavelengths other than the desired EUV wavelength are termed “out-of-band” wavelengths that, if not removed, can cause various problems including an undesirable amount of heating of the EUV optics, the reticle, the photoresist on the lithographic substrate (e.g., wafer), and the lithographic substrate itself. Although most of the EUV radiation that is produced by the plasma and that is outside the specified EUV exposure bandwidth would be absorbed by the mirrors of the illumination-optical system, extraneous wavelengths of EUV light can cause exposure problems such as image blurring. Consequently, for exposure the EUV light desirably is substantially limited to the specified wavelength. Further blurring of the image in the photoresist can occur from out-of-band deep ultraviolet (DUV) radiation that can expose the photoresist as well.
A conventional SPF is configured (see details in next paragraph below) so as to block as much of the out-of-band wavelengths as possible, including longer wavelengths (IR, UV, visible) of light and unwanted wavelengths of EUV radiation. Also, if interposed between the plasma and the illumination-optical system, the SPF serves as a physical barrier that at least slows down the rate at which debris and gases from the plasma migrate to the illumination-unit chamber and beyond. Thus, the SPF helps prevent at least some of the gases and debris from contaminating, degrading, or otherwise damaging the EUV-optical elements of the illumination-optical system. Also, because the SPF blocks longer wavelengths of radiation, it reduces heating of EUV-optical elements located downstream of the SPF, and thus reduces thermal deformation of the downstream EUV-optical elements, thereby improving their imaging performance. But, because EUV transmission through a conventional SPF decreases with increasing thickness of the SPF, the SPF must be very thin to provide adequate transmission of the desired EUV wavelength.
The conventional SPF is an approximately 100-nm thick foil of zirconium (Zr); the foil is produced by vacuum deposition and gently laid onto and bonded to a supporting mesh of wires (e.g., nickel). See, e.g., Powell, “Care and Feeding of Soft X-ray and Extreme Ultraviolet Filters,” Proceedings SPIE 1848:503, 1992; Powell et al., “Thin Film Filter Performance for Extreme Ultraviolet and X-ray Applications,” Optical Engineering 29(6):614, 1990. Other candidate materials for the foil are niobium (Nb), ytterbium (Y), and silicon (Si), but these materials are less favored. Si exhibits good transmission at 13.4 nm (0.84 for a 100-nm thick film), but the transmission through this material also is high for other EUV wavelengths, so it does not exhibit much discriminatory filtering. Nb exhibits lower transmission at 13.4 nm than Si, and Y has problems with oxidation (as does Zr).
Conventional SPFs also have other disadvantages. First, although the Zr foil is very thin, it still absorbs approximately 30% of the incident 13.4-nm EUV radiation. No practical way has been found to make the Zr foil significantly thinner (to increase desired EUV transmission), especially without seriously compromising its mechanical integrity. Second, a conventional SPF is extremely fragile. Increasing its strength and durability by increasing the thickness of the Zr foil is not practical because increased foil thickness blocks even more incident EUV transmission. Third, the wire mesh must be coarse to minimize absorption by the mesh of a substantial fraction of the incident EUV radiation. As a result, much of the Zr foil (spanning open regions of the mesh) is only weakly supported. Fourth, since a substantial fraction of the radiation produced by the plasma is out-of-band, the SPF may have to absorb a large amount of power, which causes substantial heating of and possible damage to the SPF. Fifth, depending upon the nature of any debris-mitigation system upstream of it, the SPF may be vulnerable to erosion or deposition damage as well as additional heating from particles emitted from the plasma. Sixth, while it is advantageous to locate the SPF near the intermediate focus plane, the intense plasma and radiation intensities produced in commercial EUV lithography tools are likely to damage the SPF quickly. Seventh, the opaque supporting mesh of the SPF limits location of the SPF to regions of the illumination-optical system where the mesh is substantially out of focus at the reticle plane. Otherwise, the mesh would affect the EUV illumination uniformity at the reticle and thus at the wafer, creating imaging problems.
Whereas the conventional SPF summarized above has utility in the laboratory-scale EUVL systems developed to date, which have been operated with relatively low-intensity EUV beams, conventional SPFs may fail when subjected to the substantially higher-power EUV beam produced in the near future by a commercial-scale EUVL system. Thus, there is a need for SPFs that do not have the many disadvantages of conventional SPFs.